This invention relates to an improved process for reforming of paraffins to aromatics which uses a catalyst comprising L zeolite a Group VIII noble metal, and rhenium. The ratio of Group VIII noble metal to rhenium is from about 0.1:1 to about 10:1. The unsulfided catalyst of this process shows improved catalyst life, better sulfur tolerance and improved product selectivity compared to known L-zeolite catalysts.
Catalytic reforming is used in the petroleum industry for converting paraffins to aromatic hydrocarbons. This reaction can be used, for example, for increasing the octane of gasolines, or producing aromatic feedstocks for chemical or plastics industries. Typically in this process, the naphtha is passed over a suitable catalyst under reforming conditions. The process involves several types of reactions, including isomerization, dehydrocyclization of paraffins to produce naphthenes and aromatics, dehydrogenation of cyclohexanes and other naphthenes and alkanes, isomerization or dehydrogenation of cyclopentanes, isomerization of normal paraffins to isoparaffins, and hydrocracking. The most important reforming reactions are those which produce aromatics, particularly dehydrocyclization. It is the increased aromatics content which increases the octane value of naphtha to make it a useful gasoline blending component. Conventional reforming catalysts contain a noble metal, usually platinum, on an inorganic oxide carrier material such as silica, alumina or silica-alumina. Natural or synthetic zeolites such as mordenite, X, Y, and L have also been used. Catalysts available commercially use gallium, rhenium, tin, iridium, and other metals to alter the properties of reforming catalysts in specific ways.
The addition of rhenium to conventional platinum reforming catalysts is well known and well documented. Rhenium addition substantially increases catalyst life in the reforming process, allowing longer run times between shutdowns, less money spent on catalyst and/or higher severity operations. Typically, when added to conventional non-zeolite, noble metal reforming catalyst, rhenium has no effect on the catalyst selectivity, but it does reduce the effects of catalyst deactivation. It also makes the catalyst more sensitive to sulfur poisoning. The advantages of the lower deactivation rate can be negated by the sulfur poisoning effects when the feedstock sulfur levels exceed a few parts per million.
The addition of rhenium to conventional reforming catalysts also necessitates the use of special start-up procedures. Fresh platinum-rhenium catalysts have extremely high initial activity, and result in very low amounts of liquid products unless steps are taken to limit the catalyst activity. Conventional platinum-rhenium catalysts need to be presulfided prior to use in a reforming process. For example, presulfiding Pt/Re on alumina catalysts is discussed in Shum, V. K., Butt, J. B. and Sachtler, W. M. H. "The Effects of Rhenium and Sulfur on the Activity Maintainence and Selectivity of Platinum/Alumina Hydrocarbon Conversion Catalysts" Journal of Catalysis Vol. 96, 1985, pp. 371-380. The authors state "The initial activity of the sulfided catalyst is lower than that of the unsulfided catalyst, as should be expected, because of the poisoning effect of sulfur. It is most remarkable, however, that the sulfided catalyst displays a much smaller activity decline so that this catalyst shows a higher total conversion after several hours of reaction and retains this superior activity for the duration of our experiments." It is believed that the fresh catalyst is so active that it needs to be temporarily poisoned to reduce undesirable reactions until some coke buildup on the catalyst moderates the activity. In practice, it is very difficult to achieve the exact amount of sulfur addition to reduce the initial activity surge without doing other damage to the catalyst. Insufficient sulfur does not adequately suppress the hydrogenolysis reactions. Excess sulfur poisons the catalyst for a longer time than necessary, or it washes onto catalyst of other reactors causing them to lose activity as well. Presulfiding is normally accomplished by passing a gaseous stream comprising hydrogen sulfide over the catalyst bed before the hydrocarbon feedstock is introduced. Often this is done in-situ, when the catalyst has been loaded into the reactor, but before the reactor is put on stream. The presulfiding step can also be done as a separate step, before the catalyst is loaded into the reactor. Conventional platinum-rhenium catalysts are always presulfided before introduction of the catalyst feedstock. In a cyclic process where the catalyst can be regenerated as often as every 3-4 days, presulfiding has been used with each reactor as it is put back on stream to control unwanted reactions as the feedstock is introduced.
Zeolite L is well known in the patent literature as a reforming catalyst. U.S. Pat. No. 4,634,517, issued to Tauster et al., claims a process for reforming hydrocarbons which uses a catalyst comprising zeolite L in which 75% of the exchangeable cations are from a group including potassium and barium. The catalyst also contains a Group VIII noble metal which is finely dispersed over the catalyst surface. This reference teaches that rhenium could be added to the catalyst if a noble metal is present. The reference contains no data or teachings as to what the effect of the rhenium addition would have, if any. The reference also teaches that introducing a small amount of sulfur to the catalyst is desirable to minimize the hydrocracking reactions that are present at the beginning of the reaction.
Potassium exchanged Zeolite L is claimed in a dehydrocyclization process in U.S. Pat. No. 4,614,834, issued to Lambert et al.. The process is carried out under reforming conditions. The catalyst contains platinum or another Group VIII metal component. This reference also teaches that metal components known to have modifying properties, including rhenium, may be added to the catalyst. Again there are no data or examples which show the use of rhenium to demonstrate what effect, if any, addition of rhenium to this catalyst composition may produce.
U.S. Pat. No. 4,456,527 discusses how a lack of stability due to sulfur sensitivity is a particular problem with catalysts comprising large-pore zeolites, such as L zeolite. The sulfur levels required when using such a catalyst are an order of magnitude or more below the levels permissible for even the most sulfur-sensitive conventional reforming catalysts. This reference suggests several alternatives for reducing the feedstock sulfur level to the 500 parts per billion level, but none include modifications to the catalyst itself.
Improved sulfur tolerance is accomplished by Pandey et al. in U.S. Pat. No. 4,680,280 by the addition of a desulfurization metal to a reforming catalyst comprising zeolite L and platinum. A platinum-molybdenum on Zeolite L catalyst is shown to have longer catalyst life compared to a non-molybdenum-containing catalyst. These catalysts were tested, however, at a feedstock sulfur level of only 0.2 ppm, or less than half the sulfur level recommended for Pt/Zeolite L catalysts.
Poeppelmeier et al., in U.S. Pat. No. 4,648,960 disclose a reforming process which uses a catalyst comprising a binder, a type L zeolite, exchangeable cations and a Group VIII noble metal, where the noble metal is well dispersed over the surface of the catalyst and the particles containing the noble metals are less than 7 Angstroms in diameter. This patent also contains a broad teaching that rhenium may be present, but there are no examples demonstrating the use of rhenium, nor are there teachings as to what amount of rhenium are useful.
In U.S. Pat. No. 4,595,670, Tauster et al. claim a catalyst comprising a type L zeolite, a Group VIII metal, with good dispersion of the noble metal and noble metal-containing particles of less than 7 Angstroms in diameter. Again, there is a teaching that rhenium can be present, but there are no examples on its use. There are no teachings as to what level of rhenium is useful.
A dehydrocyclization process is claimed by Lambert et al. in U.S. Pat. No. 4,746,764, which uses a catalyst comprising a type L zeolite, a Group VIII noble metal, and a surface deposited alkali metal with an alkali metal index of from 40 to 500, and which is made without subjecting the L zeolite to a pH greater than 9. This patent also contains a teaching that metals known to modify reforming catalysts, rhenium is listed, may be used in the invention. It is taught that "catalytically effective amounts of such modifiers" can be used in the invention. There are no examples containing rhenium to demonstrate a "catalytically effective amount".
U.S. Pat. No. 4,448,891, issued to Cohen, claims a catalyst for reforming produced by soaking a type L zeolite prior to calcining the dehydrocyclization metal-loaded zeolite in an alkali solution having a pH of at least 11. This patent also lists common metals used to modify reforming catalysts, such as rhenium, tin, iridium or germanium. There are no operating examples using rhenium, nor any teaching of the amount to be used.